Heretofore, field coils have been retained in place .Iadd.in various ways including .Iaddend.strips of metal.[., tape or cord which are bent or tied around the turns at opposite ends of the core and which bear against the outer surface of the core, but are not secured to it in any manner and consequently do not securely anchor the coils to the core..]. .Iadd.which are secured to the core by rivets passing through holes formed in the core and strips. These strips must be insulated and this has been done by either placing a sleeve of insulating material around them before they are riveted in place or by spraying them with an epoxy resin after securing them to the core and at the same time that the rest of the core is sprayed. Also, the rivet heads must be insulated as they are contacted by the field coils and the rivets are in conductive contact with the stack. If the strips are insulated by sleeves, separate means must be employed to insulate the rivet heads. If the core and strips are insulated by spraying epoxy resin, this would also insulate the heads, but this type of insulation would be damaged by the subsequent bending of the strips around the coils. .Iaddend..[.Moreover, such retaining means have not been put in place until after the coils have been wound and the winding guides removed with the result that some of the turns may cascade, that is fall out of place..].